The research team synthesized their catalytic material using a meticulously controlled hydrothermal process to produce HZSM-5 zeolites, subsequently ion-exchanged with iron acetylacetonate (Fe(acac)3) to embed iron species within the zeolite framework. This was followed by a sequence of stirring, washing, drying, and high-temperature calcination at 800 °C, which yielded a robust Fe@ZSM-5 catalyst. Advanced characterization techniques, including X-ray diffraction (XRD) and transmission electron microscopy (TEM), confirmed the preservation of the MFI zeolite structure and revealed well-distributed Fe2O3 nanoparticles prominently exposing (110) and (104) crystal facets on the zeolite surface.

Delving deeper into the catalyst’s composition, energy-dispersive X-ray spectroscopy (EDS) mapping and aberration-corrected scanning transmission electron microscopy (AC-STEM) elucidated a dual presence of atomically dispersed iron atoms alongside ~1.5 nm iron oxide nanoparticles. Electron energy loss spectroscopy (EELS) further verified the dominance of Fe3+ oxidation states within the catalyst, implicating this as a key feature governing catalytic behavior. X-ray photoelectron spectroscopy (XPS) detected both Fe2+ and Fe3+ species, while the O 1s spectra indicated that framework oxygen substantially contributes to the catalyst’s surface chemistry, vital for SCR reactions.

Surface acidity and reducibility are critical parameters influencing catalytic performance. Through ammonia temperature-programmed desorption (NH3-TPD) and hydrogen temperature-programmed reduction (H2-TPR), the study demonstrated that increasing iron loading enhances surface acid sites and facilitates the reduction from Fe2O3 to Fe3O4 while suppressing reduction to metallic Fe phases. These findings coincide with the predominance of Fe3+ species under reaction conditions, maintaining an optimal balance between active site availability and structural stability. Complementary X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) analyses revealed an increase in Fe–Fe coordination contacts as iron loading rose, indicating a gradual shift toward bulk iron oxide phases that correlate with catalytic activity trends.

The catalytic evaluation of Fe@ZSM-5 for NH3-SCR reactions unveiled a fascinating temperature-dependent dual kinetic regime. Optimal high-temperature NO conversion was observed with a Si/Al ratio of 27, reaching up to 95.1% NO conversion within the 400–700 °C temperature window. Above 700 °C, however, NO conversion declined markedly, especially with higher iron loadings, attributed to escalating ammonia oxidation competing pathways. This over-oxidation reduced the overall NOx reduction efficiency, confirming that precise control of operational parameters and catalyst composition is paramount for maximal efficacy.

Interestingly, gas hourly space velocity (GHSV) studies revealed an inverse relationship between feed flow rates and NO conversion, mirrored by a commensurate drop in NH3 conversion, suggesting diffusion limitations and kinetic constraints influencing reaction pathways at high throughput conditions. Durability tests at 700 °C extending beyond 50 hours showcased extraordinary catalyst stability, with the 0.1Fe@ZSM-5 variant only experiencing a minor 2.5% decrease in NO conversion, affirming its potential for practical long-term applications under harsh industrial environments.

The real-world pertinence of the catalyst was further tested under challenging conditions incorporating 300 ppm sulfur dioxide (SO2) and 8.3 vol% water vapor, simulating flue gas compositions. NO conversion initially dropped from 83.0% to 60.1% over 50 hours but intriguingly exhibited partial recovery to 71.5% once the poisoning agents were removed. This resilience contrasts sharply with sulfur-induced irrecoverable deactivation observed in parent HZSM-5 catalysts. The researchers pinpointed sulfur deposition, framework dealumination, loss of Lewis acid sites, and lattice oxygen consumption as the multiple intertwined factors responsible for the observed deactivation, providing insight for future catalyst improvements.

A pivotal advancement in this work lies in the development of a kinetic model that captures the dual-reactive pathways inherent in high-temperature NH3-SCR processes. By integrating NH3 oxidation dynamics alongside NOx selectivity transitions, the model successfully describes the experimental phenomenon where NO formation surpasses dinitrogen generation at elevated temperatures. In situ diffuse reflectance infrared Fourier-transform spectroscopy (DRIFTS) validated the emergence and role of NH2* intermediates central to the SCR mechanism, particularly under high-temperature conditions, corroborating the theoretical framework proposed.

Crucially, the size of iron nanoparticles exerts a significant influence on the reaction mechanism. Larger Fe particles enriched in metallic Fe0 species were found to enhance NH3 adsorption on Brønsted acid sites, catalyzing increased ammonia over-oxidation to NO at elevated temperatures. This size-dependent modulation creates a delicate balance governing catalytic performance, underscoring the importance of controlling iron dispersion and particle dimensions during catalyst synthesis to optimize SCR activity and minimize undesired side reactions.

The multidisciplinary team comprising Xinlin Xie, Jibin Yuan, Lei Liu, Hanzi Liu, and Zhiqiang Sun combined expertise in materials chemistry, surface science, and catalysis to deliver this comprehensive study. Their efforts were supported by the National Natural Science Foundation of China and the Provincial Natural Science Foundation of Hunan, highlighting the vital role of sustained funding in advancing frontier research tackling energy and environmental challenges.

This work not only advances fundamental understanding of iron-based catalysts under extreme reaction conditions but also propels industrial applications aimed at mitigating NOx emissions, a critical component of air pollution control strategies worldwide. By unraveling the complex kinetic interplay between NH3 oxidation and NO reduction pathways, and engineering tailored Fe@ZSM-5 catalysts with exceptional stability and activity, this research sets a new benchmark for the design of next-generation SCR catalysts capable of enduring rigorous operational demands while delivering superior environmental performance.

As the global community intensifies its commitment to reducing pollutant emissions and transitioning toward cleaner technologies, breakthroughs such as this illuminate pathways to more effective catalytic materials. Their implications span across automotive exhaust treatment, power generation, and chemical manufacturing sectors, contributing to more sustainable industrial practices and improved air quality.

Subject of Research: High-temperature selective catalytic reduction of NOx using ammonia over iron-modified ZSM-5 catalysts and kinetic modeling of competing reaction pathways.

Article Title: Dual kinetic effect from confined iron nanoparticles in zeolite modulates high-temperature catalytic NO reduction and NH3 oxidation.

News Publication Date: 15-Dec-2025.

Web References:

• Industrial Chemistry & Materials Journal
• DOI: 10.1039/D5IM00245A

Image Credits: Zhiqiang Sun, Central South University, China.

Keywords

Iron-based catalysts, high-temperature NH3-SCR, NOx reduction, Fe@ZSM-5, ammonia oxidation, catalytic mechanism, kinetic modeling, zeolite, iron nanoparticles, environmental catalysis, catalyst stability, selective catalytic reduction

The core challenge in electrocatalysis for hydrogen production lies in optimizing the interplay between catalyst activity, durability, and mass transport at the gas-liquid-solid interface. Porous materials, especially carbon frameworks, have long been leveraged for their vast surface areas and interconnected channels, which facilitate catalytic sites’ accessibility. However, disordered pore architectures often induce irregular gas flow and electrolyte transport, resulting in uneven reaction kinetics, bubble accumulation, and structural degradation over extended operation. This latest study overcomes these barriers by carefully orchestrating pore order and surface chemistry at the nanoscale.

Central to their approach was the application of finite element simulation to rigorously analyze gas pressure distributions in different porous environments. The team found that ordered mesoporous structures facilitate uniform gas flow and consistent impact stress on pore walls, in contrast to disordered networks where gas velocity varied widely between large and small pores. These pressure disparities were found to disrupt the stability of the thin electrolyte film at the catalytic interface, reducing effective contact area and catalytic activity. By promoting a stable, ultra-thin liquid membrane along hydrophilic pore surfaces, the ordered framework maximizes gas-liquid-solid equilibrium and mass transfer kinetics, a vital advancement for efficient hydrogen evolution reactions.

Building on this theoretical insight, the researchers devised the superlattice blotting synthesis to translate simulations into functional materials. They employed the self-assembly of nickel nanocrystals into superlattices, effectively confining oxidative reactions and enhancing thermal stability without compromising nanoscale morphology or size. Subsequent ligand carbonization at moderate temperatures preserved the superlattice’s porous architecture, while acid etching exposed a robust three-dimensional mesoporous carbon skeleton enriched with nickel ions. A final high-temperature graphitization step incorporated heteroatom doping, such as nitrogen, sulfur, and phosphorus, fine-tuning electronic properties and overall conductivity.

Advanced characterization techniques including spherical aberration electron microscopy and extended X-ray absorption fine structure spectroscopy confirmed the successful dispersion of isolated nickel single atoms within the mesoporous carbon matrix. The nickel centers were coordinated distinctly in two catalyst variants: Ni atoms bonded to two nitrogens and two sulfurs (Ni-N₂S₂) and Ni atoms coordinated with three nitrogens and one phosphorus atom (Ni-N₃P). This precise heteroatomic coordination was shown to play a critical role in modulating the electronic environment of the metal centers and optimizing catalytic function.

Evaluating electrocatalytic performance revealed remarkable results. The Ni-N₂S₂ catalyst demonstrated superior oxygen evolution reaction (OER) activity, achieving an impressively low overpotential of 239 millivolts at 20 milliamps per square centimeter, significantly outperforming commercial ruthenium oxide catalysts under the same conditions. Conversely, the Ni-N₃P configuration excelled in the hydrogen evolution reaction (HER), requiring a mere 90 millivolts overpotential to reach 10 milliamps per square centimeter, indicative of rapid kinetics consistent with a Heyrovsky-type mechanism. These values rank among the best reported for nickel-based single-atom catalysts.

Further leveraging these synergistic catalysts, the team assembled a two-electrode system pairing the Ni-N₂S₂ catalyst as the anode with the Ni-N₃P catalyst as the cathode, achieving full water electrolysis at a low overall cell potential of 1.59 volts for 10 milliamps per square centimeter. This system maintained stable operation for over 100 hours, demonstrating exceptional durability without compromise in catalytic current, a key criterion for practical renewable energy applications. The ordered porous support is critical to this performance, facilitating efficient gas release and electrolyte circulation, thereby maintaining an active three-phase boundary.

This study marks a major leap forward in catalysis design by bridging theoretical models, innovative synthesis routes, and precise atomic engineering. The integration of finite element-guided superlattice imprinting with heteroatom-coordinated single-atom catalysts offers a blueprint for developing durable, highly active electrocatalysts that harness the full potential of nanoconfinement and optimized microenvironments. Moreover, the formation of stable ultra-thin liquid membranes along hydrophilic ordered pore walls highlights a fundamentally new mechanistic understanding of three-phase electrochemical interfaces.

The implications extend beyond water splitting, as the principles and methodologies demonstrated here can be adapted to a wide range of electrocatalytic transformations, including CO₂ reduction, nitrogen fixation, and fuel cell reactions. The high thermal stability and electronic tunability arising from the superlattice-derived ordered mesoporous skeleton open pathways for tailored catalyst architectures with expanded functionality. This cross-disciplinary approach combining computational modeling, nanoengineering, and advanced characterization sets a new standard for catalyst innovation.

Looking forward, the team’s framework for constructing high-density nickel single atoms anchored inside tailored mesoporous hosts could drive the next generation of clean energy devices. Coupled with ongoing developments in scalable manufacturing and renewable feedstocks, these fundamental breakthroughs promise to accelerate the global transition toward green hydrogen production and sustainable chemical synthesis. With continuing research, the underlying concepts of controlled three-phase equilibrium and precise heteroatom coordination are poised to become ubiquitous tools in the design of future electrocatalytic systems.

This pioneering research not only amplifies the functional capabilities of carbon-based catalysts but also exemplifies the power of integrating computational design with experimental realization. Such synergy enables the creation of complex yet finely controlled materials at the atomic scale, bridging the gap between fundamental science and practical technologies. As the urgency of climate change intensifies, innovations like these are critical to unlocking the potential of renewable energy and catalysis for a sustainable future.

The study was published in CCS Chemistry, the flagship journal of the Chinese Chemical Society, reflecting the global importance and interdisciplinary nature of the work. With the no-cost open-access policy of the journal, these insights and technological advances are now accessible to the worldwide scientific community, fostering collaboration and rapid progress in electrocatalysis and energy materials science. The authors declare no conflicts of interest, emphasizing the academic rigor and integrity underpinning these contributions.

In summary, by leveraging finite element simulations to inform the design of ordered mesoporous carbon skeletons and deploying a novel superlattice blotting synthesis, this work delivers a blueprint for fabricating stable, high-performance nickel single-atom electrocatalysts. The stable gas–liquid–solid interface supported by an ultra-thin liquid film, combined with heteroatom-tuned single-atom sites, achieves unrivaled catalytic efficiency and durability in water splitting. These advances bring us closer to scalable, economically viable hydrogen production, a cornerstone in the quest for sustainable energy systems worldwide.

Subject of Research: Not applicable

Article Title: A Superlattice Imprinting Method to Construct High Ni Single Atoms Inside Ordered Mesoporous Carbon for Efficient Electrocatalysis

News Publication Date: 10-Sep-2025

Web References:
https://www.chinesechemsoc.org/journal/ccschem
http://dx.doi.org/10.31635/ccschem.025.202505938

References:
Yuanyuan Wang, Wenlei Zhu, Yuehe Lin et al., CCS Chemistry, DOI: 10.31635/ccschem.025.202505938

Image Credits: CCS Chemistry

Keywords

Superlattices, ordered mesoporous carbon, single-atom catalysis, nickel single atoms, electrocatalysis, hydrogen evolution reaction, oxygen evolution reaction, finite element simulation, heteroatom doping, superlattice blotting, energy conversion, water splitting